The fixed or fatty oils, although varying considerably in external appearance, form in reality a well-defined and homogeneous group of substances having great similarity of chemical composition. They appear to be essential constituents of the most highly-organized forms of animal and vegetable life, being found in plants chiefly in the seed, and in animals chiefly enclosed in the cellular tissue and in special body cavities, but some proportion of fatty matter is found in almost all tissues and organs. Although oils and fats are universally distributed and perform most important functions in animal and vegetable life, those used for technical purposes are not drawn from any very great number of sources; and many bodies might be utilized for the production of oil which at present are not so employed.

As found in commerce, oils possess a faint characteristic taste, a slight odour, and some amount of colour, generally brownish yellow. These characteristics, however, are due to certain impurities; in a really pure condition most oils have scarcely any characteristic taste, odour, colour, or physiological influence. In a few cases only they have special properties which appear to be inseparable characteristics, such as the purgative principle of castor oil, croton oil, and some others. At the ordinary temperatures most vegetable oils are fluid, but a few, produced especially by tropical plants, such as palm oil, cocoa butter, Chinese tallow, Ac., are solid fats. Animal fats are for the most part solid, the oils of marine animals and neat’s-foot oil being important exceptions. The various solid fats differ greatly in consistency, and the hardness of individual samples is largely affected by the nature of the food and by the health of the animal yielding them, and by some other circumstances. The relative fluidity or solidity of the various oils and fats depends on the proportions of the three principal constituents of all oils—olein, stearin, and palmitin. The fluid oils contain olein in larger proportion, that body being itself liquid at ordinary temperatures, while solid stearin and palmitin predominate in the hard fats. The viscosity of the fluid oils also ranges between wide limits. The rate of flow of an oil, which is a matter of considerable importance in several industrial applications, is estimated by comparison with the standard rate at which water of the same temperature flows through an aperture of fixed dimensions. The most viscid of the fluid oils is castor oil, which at a temperature of 15° C. is more than two hundred times thicker or more slow-flowing than water. Olive oil at the same temperature is more than twenty times thicker, and linseed and hemp oils, though among the most limpid of fixed oils, still flow about ten times as slowly as water.

Oils communicate to paper and like substances a stain which remains an irremovable translucent grease spot. They are almost entirely insoluble in water, and, excepting croton and castor oils, in cold alcohol, but in boiling alcohol they dissolve more freely, and they are perfectly soluble in ether, bisulphide of carbon, chloroform, benzol, and light petroleum spirit. In their pure condition they are neutral bodies, but, on their becoming rancid, free fatty acids are developed which give them an acid reaction. Exposed to air they absorb oxygen freely, and the class containing linoleic acid, known as drying oils, of which linseed oil is the type, thereby harden into a solid translucent semielastic caoutchouc-like body, a property of the utmost value in the arts. When they are exposed in thin layers over a great surface the absorption of oxygen proceeds with such energy that heat is evolved sufficient to produce spontaneous combustion, a circumstance frequently exemplified in the heating and igniting of heaps of oily cotton waste. The non-drying oils also on exposure to air thicken and become greasy; they acquire the peculiar disagreeable smell and acrid taste known as rancidity, owing to a kind of fermentation being set up in them through the agency of impurities, whereby the fixed fatty acids they contain are decomposed, and odorous volatile fatty acids formed by oxidation at their expense.

The specific gravity of all oils is lower than that of water, ranging from '900 in the case of cocoa butter to ∙970, the specific gravity of castor oil. Most fluid oils have a specific gravity between ’ 915 and ∙930. The specific gravity of oils varies with the temperature far more than is the case with water. It is found that for each degree Centigrade rise of temperature whale oil increases in volume 1 per 1000, rape oil 0 ’ 89, and olive oil 0’83.

When a solid fat is heated slowly till it melts and is allowed gradually to cool, it remains fluid till it falls considerably under the temperature at which it melted, and at the moment of solidification there is a sensible increase in its temperature. Butter, for example, melts at 30° to 31 0, 5 C., but does not resolidify till it falls to 19° or 20° C. These phenomena have been investigated in the case of the pure fats stearin and palmitin by Duffy (Chem. Soc. Qu. J., V. 197), who finds that these bodies undergo with great readiness three isomeric modifications, each having a distinct melting point widely apart from each other (stearin from beef giving 5Γ, 63°, and 67°), the solidifying point being slightly under the lowest of the three. The freezing point of the ordinary fluid oils ranges down to from - 27° to - 28 β C. for hemp oil, nut oil, and linseed oil, while olive oil solidifies at + 2° to 4° C. Fluid oils heated to from 280° to 300° C., and solid fats to from 300° to 325° C., undergo destructive distillation, resolving into a mixture of rich inflammable gases and a peculiarly irritating acrid vapour, acrolein.

Oils and fats are compounds of carbon, hydrogen, and oxygen, in proportions ranging, as a rule, for carbon between 76 and 80 per cent., hydrogen from 11 to 13 per cent., and oxygen from 10 to 12 per cent. Their proximate constitution was first demonstrated by Chevreul, who indeed, in the great series of classical researches embodied in his Recherches sur les corps gras iïorigine animale (1823), established the modern chemistry of oils.

The phenomena of saponification, as exemplified on a great scale in the important industry of soap-making, furnished the key for discovering the intimate constitution of oils. Oils and fats treated with alkalis, alkaline earths, and basic metallic oxides in presence of water undergo decomposition and enter new combinations. A soap is formed by the union of the alkaline body with acid constituents of the oil, known as fatty acids, ajιd the sweet body, glycerin, is liberated. The saponification of stearin with sodic hydrate, for example, may be thus represented :—

c'⅛° h } o, + 3 g≈} o = 3 ⅛⅞θ } o + c ¾"'} 0,.

Stearin. Sodic Stearate of Glycerin, hydrate, sodium (soap).

By heating oil with steam under a pressure of from 10 to 12 atmospheres, or with water superheated to about 220° C., the oil is decomposed into free fatty acid and glycerin. Thus, again taking the simple fat stearin, we have :—

⅛⅝ o 4o 3 + 3 ¾ o = 3c ½θ} 0 + Cs §'"}0 3 .

Stearin. Water. Stearic acid. Glycerin.

In the above reactions it will be observed that three molecules of water are required for the formation of free stearic acid and glycerin from one molecule of stearin, and to that extent the resulting products are heavier than the original. Beading the equation in the inverse manner we find the formation of stearin takes place by the substitution of the three acid residues of stearic acid C 18 H 35 O for the three hydroxyls H. O in the molecule of glycerin. No fat or oil is found in nature consisting of a single chemical fat such as stearin alone. All are mixtures of at least two and for the most part three or·more simple fats or glycerides of fatty acids closely allied in nature and constitution. These glycerides or combinations of glycerin and fatty acids are in their chemical relations ethers. Glycerin itself is a triatomic alcohol, and bears to the fatty acids and resulting ethers the same relation which a basic substance bears to an acid and to the salt which results from their combination. In all natural fats glycerin combines, as in stearin, by having substituted for its three replaceable hydrogen atoms three equivalents of fatty acids, whence the natural simple fats are all triacid compounds — tristearin (θi8¾5θ)3 j. O 3 , tripalmitin (θi6¾1θ)310 3j triolein (θi8¾O9 3 j. 0 3 , &c., though commonly called stearin, palmitin, and olein, &c. (For further information as to the constitution of glycerides see Glycerin, vol. x. p. 697.) The three simple fats above named form by far the largest and most important constituents of all oils and fats, the only others which bulk largely being the glycerides of linoleic acid in drying oils, and of physetoleie acid characteristic of marine oils. The number of fatty acids found combined with glycerin in oils is, however, very considerable. They constitute members of homologous series, the first or stearic series of which possess the common formula C n H 2n O s . Belonging to it are the following :— [table]

Another, the oleic series, contains two atoms less of hydrogen than the allied stearic scries, having the general formula C n H 2n . 2 O 2 . To it belong the following :— [table]

To these there remain to be added, not members of either series, two important fatty acids, —linoleic acid, C 16 H 28 O 2 , characteristic of linseed oil and other drying oils, and ricinoleic acid C 18 H 34 O 3 , the chief product of the saponification of castor oil. Among saponifiable bodies the true waxes are distinguished from other solid fats by containing no glycerin. They are principally ethers of the higher monatomic solid alcohols—cetylic and cerylic alcohol, &c. Thus spermaceti, the solid wax obtained from the head matter of the r CHOI

sperm whale, is a cetyl palmitate Certain of the vegetable waxes— e.g., Japanese wax—contain some proportion of glycerides.

Extraction of Oil.

The ordinary method for separating vegetable oils and fats from the nuts, seeds, &c., of which they form constituent parts is by pressure, with or without the assistance of heat. They are also obtained by the agency of solvents, principally by the use of bisulphide of carbon and the light petroleum spirit benzin, these being methods of production of comparatively recent introduction. Animal oils and fats are principally isolated by simple melting or “rendering” by heat. The degrees of heat and pressure necessary for separating the several fats vary very much with the fluidity of the oils themselves, the proportion in which they are present in the substances, and the nature and consistence of the associated materials. Sperm oil is indeed obtained direct in its fluid condition from the head of the sperm whale. Virgin olive oil is obtained with the gentlest pressure, and palm oil and several other vegetable fats and waxes are liberated by the agency of boiling water.

Vegetable Oil Pressing.— Pliny describes in detail the apparatus and processes used for obtaining olive oil among his Roman contemporaries, by which it appears that they derived a knowledge of the screw press from the Greeks, and applied it to the pressure of oil from pulped olives. In the East, where vegetable oil forms a most important article for food and for other personal and domestic purposes, various ingenious applications of lever presses and of combined lever and wedge presses have been in use from the earliest times. The Chinese employ the same series of operations which are followed in the most advanced oil mills of modern times, viz., bruising and reducing the seeds to meal under an edge-stone, heating the meal in an open pan, and pressing out the oil in a wedge press in which the wedges are driven home by hand hammers. The apparatus used in Europe in modern times for the extraction of oil by pressure consists of forms of the screw press, the Dutch or stamper press, and the hydraulic press. With the screw press, even of the most improved form, the amount of pressure practically obtainable is limited from the failure of its parts under the severe inelastic strain which can be put on it. It is on this account only used in the pressure of olives and of animal fats, where the power necessary is not great. The Dutch or stamper press, which has played an important part in the oil industry, was invented, as its name indicates, in Holland in the 17th century. The invention of the hydraulic press in 1795 effected the greatest revolution in the oil industry, bringing a new, easily controlled, and almost unlimited source of power into play, and on the great scale that apparatus has practically superseded all other means of pressing.

The sequence of operations in treating oil-seeds for the separation of their contained oils is ordinarily as follows :—(1) the crushing and grinding of the seed or other substance, (2) heating the oleaginous meal so prepared, and (3) expression of the oil by mechanical power.

Grinding.— As a preliminary operation oil-seeds are freed from dust, sand, and other impurities by sifting in an inclined revolving cylinder or screening machine, covered with woven wire having meshes varying according to the size and nature of the seed operated upon. In earlier times the seeds were pounded to meal by means of stamper mills. These consisted of a series of heavy wooden stampers or pestles made to rise and fall by the action of cams or wypers fixed on a revolving shaft, a pair of such stampers falling alternately with heavy force into an egg-shaped mortar about two-thirds filled with seed. As the process proceeded the material became heated, and from time to time had to be sprinkled with water. Stampers are now seen only in small old-fashioned establishments. In a modern oil-mill the screened seed is passed through crushing rollers to bruise or open the husk. The crushing rollers consist of a pair of cast-iron rollers horizontally mounted, commonly of unequal size, the larger being 4 feet in diameter, while the smaller is about 1 foot The larger roller is revolved by power and the smaller moves by simple friction against the other. Between these rollers the seed is fed by a hopper, and in passing through it is bruised and broken and so prepared for the thorough grinding it receives under the revolving edge-stones to which it next passes. These are a pair of circular stones having a diameter of about 7 feet with a thickness at their running edge of 16 inches, each weighing from 2⅜ to 3⅜ tons. They are made of very compact limestone, granite, or fine-grained sandstone, and are mounted on a vertical driving shaft, to which they are attached by a horizontal axle passing through their centre. They revolve on a bed of similar hard compact stone, and the compound rubbing and bruising effect of their rotary motion quickly reduces the bruised seeds to a fine meal. The stones are provided with sweepers, which in their revolution bring the material pressed out towards the side again into their path, and there is a separate sweeper for clearing out the finished meal from the bed of the machine by way of a slide or door provided in the side. The edge-stones revolve about twenty times per minute, and a charge of seed which is slightly moistened during the process is sufficiently ground on an average in about twenty-five minutes.

Heating.— In dealing with certain oils which are easily separated, and especially with oils used in cookery and otherwise consumed, where it is desirable to preserve the pleasant, bland, and faint fruity or nutty taste, the ground oleaginous meal is taken direct to the press and pressed for cold-drawn or virgin oil. The cake from such cold pressing, as it still retains a large proportion of oil, is subsequently broken up, reduced to meal, and heated; after which it is again subjected to pressure to obtain a further flow of oil. Ordinarily, however, the meal is artificially heated previous to any pressure, and it depends greatly on the nature of the seed and the individual manufacturer’s method of working whether the material is fully pressed at first or twice submitted to the operation. The warming of the seed meal renders the contained oil more fluid and consequently more readily separable with moderate pressure. It also enables the oil-presser to obtain a larger proportion of the contained oil, coagulates and holds back the albuminous constituents of the seeds, and similarly dries and retains mucilaginous matter. On the other hand, oil from heated meal usually is more highly coloured and harsher to the taste than cold-drawn oil; and the quality is seriously deteriorated if by chance the heat applied should exceed at most 80° C∙ The heat is applied either in open shallow iron kettles or pans heated over a direct fire or through a sand-bath; but preferably, and now generally, the meal is heated by steam circulating freely between the casings of a jacketed or double-walled pan or pans. Mechanical stirrers are kept in continuous rotation within the pan, to ensure a uniform warming effect throughout the mass. A highly-approved and convenient form of heating apparatus consists of a double steam kettle, one pan being placed above the other, each steam-jacketed and provided with mechanical stirrers. In this machine the heating action is continuous. The meal is first heated for ten or fifteen minutes in the upper pan, which is closed over with a sheet-iron cover, after which a slide in the bottom of the pan is opened and the charge is shot down into the lower pan, where it is raised to the full heat, while the upper pan is again recharged and worked up. When fully heated in the lower pan the charge is swept out at a door in the side of the pan by the action of the mechanical stirrers, and falling into a funnel is passed in measured quantities direct into bags, and without delay prepared for and placed in the press. The kettles are of a capacity sufficient to heat seed for charging three single presses at each operation. A form of heating kettle is also now in use in which the object is effected by direct injection of steam into the mass, whereby the meal is not only heated but a beneficial amount of moisture is distributed throughout the material. In mills of the most recent construction such steam kettles are used in connexion with an improved form of crushing rollers,—employment of edge-mills being dispensed with. These rollers consist of a series of four or five chilled iron or steel cylinders mounted in vertical order like the bowls of a calender. The seeds pass in succession between the first and second rollers in the series, then between the second and third, and so on till they are delivered by the lowest, sufficiently bruised, crushed, and ground.

Pressing.— With the least possible delay the meal is transferred from the heating kettles, so that the oil may be pressed out while the material still retains its heat. Measured quantities, say 10 to 12 lb of meal, are filled into woollen bags of strong, thick texture, sufficiently open and porous to allow...

The fixed or fatty oils, although varying considerably in external appearance, form in reality a well-defined and homogeneous group of substances having great similarity of chemical composition. They appear to be essential constituents of the most highly-organized forms of animal and vegetable life, being found in plants chiefly in the seed, and in animals chiefly enclosed in the cellular tissue and in special body cavities, but some proportion of fatty matter is found in almost all tissues and organs. Although oils and fats are universally distributed and perform most important functions in animal and vegetable life, those used for technical purposes are not drawn from any very great number of sources; and many bodies might be utilized for the production of oil which at present are not so employed.

As found in commerce, oils possess a faint characteristic taste, a slight odour, and some amount of colour, generally brownish yellow. These characteristics, however, are due to certain impurities; in a really pure condition most oils have scarcely any characteristic taste, odour, colour, or physiological influence. In a few cases only they have special properties which appear to be inseparable characteristics, such as the purgative principle of castor oil, croton oil, and some others. At the ordinary temperatures most vegetable oils are fluid, but a few, produced especially by tropical plants, such as palm oil, cocoa butter, Chinese tallow, Ac., are solid fats. Animal fats are for the most part solid, the oils of marine animals and neat’s-foot oil being important exceptions. The various solid fats differ greatly in consistency, and the hardness of individual samples is largely affected by the nature of the food and by the health of the animal yielding them, and by some other circumstances. The relative fluidity or solidity of the various oils and fats depends on the proportions of the three principal constituents of all oils—olein, stearin, and palmitin. The fluid oils contain olein in larger proportion, that body being itself liquid at ordinary temperatures, while solid stearin and palmitin predominate in the hard fats. The viscosity of the fluid oils also ranges between wide limits. The rate of flow of an oil, which is a matter of considerable importance in several industrial applications, is estimated by comparison with the standard rate at which water of the same temperature flows through an aperture of fixed dimensions. The most viscid of the fluid oils is castor oil, which at a temperature of 15° C. is more than two hundred times thicker or more slow-flowing than water. Olive oil at the same temperature is more than twenty times thicker, and linseed and hemp oils, though among the most limpid of fixed oils, still flow about ten times as slowly as water.

Oils communicate to paper and like substances a stain which remains an irremovable translucent grease spot. They are almost entirely insoluble in water, and, excepting croton and castor oils, in cold alcohol, but in boiling alcohol they dissolve more freely, and they are perfectly soluble in ether, bisulphide of carbon, chloroform, benzol, and light petroleum spirit. In their pure condition they are neutral bodies, but, on their becoming rancid, free fatty acids are developed which give them an acid reaction. Exposed to air they absorb oxygen freely, and the class containing linoleic acid, known as drying oils, of which linseed oil is the type, thereby harden into a solid translucent semielastic caoutchouc-like body, a property of the utmost value in the arts. When they are exposed in thin layers over a great surface the absorption of oxygen proceeds with such energy that heat is evolved sufficient to produce spontaneous combustion, a circumstance frequently exemplified in the heating and igniting of heaps of oily cotton waste. The non-drying oils also on exposure to air thicken and become greasy; they acquire the peculiar disagreeable smell and acrid taste known as rancidity, owing to a kind of fermentation being set up in them through the agency of impurities, whereby the fixed fatty acids they contain are decomposed, and odorous volatile fatty acids formed by oxidation at their expense.

The specific gravity of all oils is lower than that of water, ranging from '900 in the case of cocoa butter to ∙970, the specific gravity of castor oil. Most fluid oils have a specific gravity between ’ 915 and ∙930. The specific gravity of oils varies with the temperature far more than is the case with water. It is found that for each degree Centigrade rise of temperature whale oil increases in volume 1 per 1000, rape oil 0 ’ 89, and olive oil 0’83.

When a solid fat is heated slowly till it melts and is allowed gradually to cool, it remains fluid till it falls considerably under the temperature at which it melted, and at the moment of solidification there is a sensible increase in its temperature. Butter, for example, melts at 30° to 31 0, 5 C., but does not resolidify till it falls to 19° or 20° C. These phenomena have been investigated in the case of the pure fats stearin and palmitin by Duffy (Chem. Soc. Qu. J., V. 197), who finds that these bodies undergo with great readiness three isomeric modifications, each having a distinct melting point widely apart from each other (stearin from beef giving 5Γ, 63°, and 67°), the solidifying point being slightly under the lowest of the three. The freezing point of the ordinary fluid oils ranges down to from - 27° to - 28 β C. for hemp oil, nut oil, and linseed oil, while olive oil solidifies at + 2° to 4° C. Fluid oils heated to from 280° to 300° C., and solid fats to from 300° to 325° C., undergo destructive distillation, resolving into a mixture of rich inflammable gases and a peculiarly irritating acrid vapour, acrolein.

Oils and fats are compounds of carbon, hydrogen, and oxygen, in proportions ranging, as a rule, for carbon between 76 and 80 per cent., hydrogen from 11 to 13 per cent., and oxygen from 10 to 12 per cent. Their proximate constitution was first demonstrated by Chevreul, who indeed, in the great series of classical researches embodied in his Recherches sur les corps gras iïorigine animale (1823), established the modern chemistry of oils.

The phenomena of saponification, as exemplified on a great scale in the important industry of soap-making, furnished the key for discovering the intimate constitution of oils. Oils and fats treated with alkalis, alkaline earths, and basic metallic oxides in presence of water undergo decomposition and enter new combinations. A soap is formed by the union of the alkaline body with acid constituents of the oil, known as fatty acids, ajιd the sweet body, glycerin, is liberated. The saponification of stearin with sodic hydrate, for example, may be thus represented :—

c'⅛° h } o, + 3 g≈} o = 3 ⅛⅞θ } o + c ¾"'} 0,.

Stearin. Sodic Stearate of Glycerin, hydrate, sodium (soap).

By heating oil with steam under a pressure of from 10 to 12 atmospheres, or with water superheated to about 220° C., the oil is decomposed into free fatty acid and glycerin. Thus, again taking the simple fat stearin, we have :—

⅛⅝ o 4o 3 + 3 ¾ o = 3c ½θ} 0 + Cs §'"}0 3 .

Stearin. Water. Stearic acid. Glycerin.

In the above reactions it will be observed that three molecules of water are required for the formation of free stearic acid and glycerin from one molecule of stearin, and to that extent the resulting products are heavier than the original. Beading the equation in the inverse manner we find the formation of stearin takes place by the substitution of the three acid residues of stearic acid C 18 H 35 O for the three hydroxyls H. O in the molecule of glycerin. No fat or oil is found in nature consisting of a single chemical fat such as stearin alone. All are mixtures of at least two and for the most part three or·more simple fats or glycerides of fatty acids closely allied in nature and constitution. These glycerides or combinations of glycerin and fatty acids are in their chemical relations ethers. Glycerin itself is a triatomic alcohol, and bears to the fatty acids and resulting ethers the same relation which a basic substance bears to an acid and to the salt which results from their combination. In all natural fats glycerin combines, as in stearin, by having substituted for its three replaceable hydrogen atoms three equivalents of fatty acids, whence the natural simple fats are all triacid compounds — tristearin (θi8¾5θ)3 j. O 3 , tripalmitin (θi6¾1θ)310 3j triolein (θi8¾O9 3 j. 0 3 , &c., though commonly called stearin, palmitin, and olein, &c. (For further information as to the constitution of glycerides see Glycerin, vol. x. p. 697.) The three simple fats above named form by far the largest and most important constituents of all oils and fats, the only others which bulk largely being the glycerides of linoleic acid in drying oils, and of physetoleie acid characteristic of marine oils. The number of fatty acids found combined with glycerin in oils is, however, very considerable. They constitute members of homologous series, the first or stearic series of which possess the common formula C n H 2n O s . Belonging to it are the following :— [table]

Another, the oleic series, contains two atoms less of hydrogen than the allied stearic scries, having the general formula C n H 2n . 2 O 2 . To it belong the following :— [table]

To these there remain to be added, not members of either series, two important fatty acids, —linoleic acid, C 16 H 28 O 2 , characteristic of linseed oil and other drying oils, and ricinoleic acid C 18 H 34 O 3 , the chief product of the saponification of castor oil. Among saponifiable bodies the true waxes are distinguished from other solid fats by containing no glycerin. They are principally ethers of the higher monatomic solid alcohols—cetylic and cerylic alcohol, &c. Thus spermaceti, the solid wax obtained from the head matter of the r CHOI

sperm whale, is a cetyl palmitate Certain of the vegetable waxes— e.g., Japanese wax—contain some proportion of glycerides.

Extraction of Oil.

The ordinary method for separating vegetable oils and fats from the nuts, seeds, &c., of which they form constituent parts is by pressure, with or without the assistance of heat. They are also obtained by the agency of solvents, principally by the use of bisulphide of carbon and the light petroleum spirit benzin, these being methods of production of comparatively recent introduction. Animal oils and fats are principally isolated by simple melting or “rendering” by heat. The degrees of heat and pressure necessary for separating the several fats vary very much with the fluidity of the oils themselves, the proportion in which they are present in the substances, and the nature and consistence of the associated materials. Sperm oil is indeed obtained direct in its fluid condition from the head of the sperm whale. Virgin olive oil is obtained with the gentlest pressure, and palm oil and several other vegetable fats and waxes are liberated by the agency of boiling water.

Vegetable Oil Pressing.— Pliny describes in detail the apparatus and processes used for obtaining olive oil among his Roman contemporaries, by which it appears that they derived a knowledge of the screw press from the Greeks, and applied it to the pressure of oil from pulped olives. In the East, where vegetable oil forms a most important article for food and for other personal and domestic purposes, various ingenious applications of lever presses and of combined lever and wedge presses have been in use from the earliest times. The Chinese employ the same series of operations which are followed in the most advanced oil mills of modern times, viz., bruising and reducing the seeds to meal under an edge-stone, heating the meal in an open pan, and pressing out the oil in a wedge press in which the wedges are driven home by hand hammers. The apparatus used in Europe in modern times for the extraction of oil by pressure consists of forms of the screw press, the Dutch or stamper press, and the hydraulic press. With the screw press, even of the most improved form, the amount of pressure practically obtainable is limited from the failure of its parts under the severe inelastic strain which can be put on it. It is on this account only used in the pressure of olives and of animal fats, where the power necessary is not great. The Dutch or stamper press, which has played an important part in the oil industry, was invented, as its name indicates, in Holland in the 17th century. The invention of the hydraulic press in 1795 effected the greatest revolution in the oil industry, bringing a new, easily controlled, and almost unlimited source of power into play, and on the great scale that apparatus has practically superseded all other means of pressing.

The sequence of operations in treating oil-seeds for the separation of their contained oils is ordinarily as follows :—(1) the crushing and grinding of the seed or other substance, (2) heating the oleaginous meal so prepared, and (3) expression of the oil by mechanical power.

Grinding.— As a preliminary operation oil-seeds are freed from dust, sand, and other impurities by sifting in an inclined revolving cylinder or screening machine, covered with woven wire having meshes varying according to the size and nature of the seed operated upon. In earlier times the seeds were pounded to meal by means of stamper mills. These consisted of a series of heavy wooden stampers or pestles made to rise and fall by the action of cams or wypers fixed on a revolving shaft, a pair of such stampers falling alternately with heavy force into an egg-shaped mortar about two-thirds filled with seed. As the process proceeded the material became heated, and from time to time had to be sprinkled with water. Stampers are now seen only in small old-fashioned establishments. In a modern oil-mill the screened seed is passed through crushing rollers to bruise or open the husk. The crushing rollers consist of a pair of cast-iron rollers horizontally mounted, commonly of unequal size, the larger being 4 feet in diameter, while the smaller is about 1 foot The larger roller is revolved by power and the smaller moves by simple friction against the other. Between these rollers the seed is fed by a hopper, and in passing through it is bruised and broken and so prepared for the thorough grinding it receives under the revolving edge-stones to which it next passes. These are a pair of circular stones having a diameter of about 7 feet with a thickness at their running edge of 16 inches, each weighing from 2⅜ to 3⅜ tons. They are made of very compact limestone, granite, or fine-grained sandstone, and are mounted on a vertical driving shaft, to which they are attached by a horizontal axle passing through their centre. They revolve on a bed of similar hard compact stone, and the compound rubbing and bruising effect of their rotary motion quickly reduces the bruised seeds to a fine meal. The stones are provided with sweepers, which in their revolution bring the material pressed out towards the side again into their path, and there is a separate sweeper for clearing out the finished meal from the bed of the machine by way of a slide or door provided in the side. The edge-stones revolve about twenty times per minute, and a charge of seed which is slightly moistened during the process is sufficiently ground on an average in about twenty-five minutes.

Heating.— In dealing with certain oils which are easily separated, and especially with oils used in cookery and otherwise consumed, where it is desirable to preserve the pleasant, bland, and faint fruity or nutty taste, the ground oleaginous meal is taken direct to the press and pressed for cold-drawn or virgin oil. The cake from such cold pressing, as it still retains a large proportion of oil, is subsequently broken up, reduced to meal, and heated; after which it is again subjected to pressure to obtain a further flow of oil. Ordinarily, however, the meal is artificially heated previous to any pressure, and it depends greatly on the nature of the seed and the individual manufacturer’s method of working whether the material is fully pressed at first or twice submitted to the operation. The warming of the seed meal renders the contained oil more fluid and consequently more readily separable with moderate pressure. It also enables the oil-presser to obtain a larger proportion of the contained oil, coagulates and holds back the albuminous constituents of the seeds, and similarly dries and retains mucilaginous matter. On the other hand, oil from heated meal usually is more highly coloured and harsher to the taste than cold-drawn oil; and the quality is seriously deteriorated if by chance the heat applied should exceed at most 80° C∙ The heat is applied either in open shallow iron kettles or pans heated over a direct fire or through a sand-bath; but preferably, and now generally, the meal is heated by steam circulating freely between the casings of a jacketed or double-walled pan or pans. Mechanical stirrers are kept in continuous rotation within the pan, to ensure a uniform warming effect throughout the mass. A highly-approved and convenient form of heating apparatus consists of a double steam kettle, one pan being placed above the other, each steam-jacketed and provided with mechanical stirrers. In this machine the heating action is continuous. The meal is first heated for ten or fifteen minutes in the upper pan, which is closed over with a sheet-iron cover, after which a slide in the bottom of the pan is opened and the charge is shot down into the lower pan, where it is raised to the full heat, while the upper pan is again recharged and worked up. When fully heated in the lower pan the charge is swept out at a door in the side of the pan by the action of the mechanical stirrers, and falling into a funnel is passed in measured quantities direct into bags, and without delay prepared for and placed in the press. The kettles are of a capacity sufficient to heat seed for charging three single presses at each operation. A form of heating kettle is also now in use in which the object is effected by direct injection of steam into the mass, whereby the meal is not only heated but a beneficial amount of moisture is distributed throughout the material. In mills of the most recent construction such steam kettles are used in connexion with an improved form of crushing rollers,—employment of edge-mills being dispensed with. These rollers consist of a series of four or five chilled iron or steel cylinders mounted in vertical order like the bowls of a calender. The seeds pass in succession between the first and second rollers in the series, then between the second and third, and so on till they are delivered by the lowest, sufficiently bruised, crushed, and ground.

Pressing.— With the least possible delay the meal is transferred from the heating kettles, so that the oil may be pressed out while the material still retains its heat. Measured quantities, say 10 to 12 lb of meal, are filled into woollen bags of strong, thick texture, sufficiently open and porous to allow...

The fixed or fatty oils, although varying considerably in external appearance, form in reality a well-defined and homogeneous group of substances having great similarity of chemical composition. They appear to be essential constituents of the most highly-organized forms of animal and vegetable life, being found in plants chiefly in the seed, and in animals chiefly enclosed in the cellular tissue and in special body cavities, but some proportion of fatty matter is found in almost all tissues and organs. Although oils and fats are universally distributed and perform most important functions in animal and vegetable life, those used for technical purposes are not drawn from any very great number of sources; and many bodies might be utilized for the production of oil which at present are not so employed.

As found in commerce, oils possess a faint characteristic taste, a slight odour, and some amount of colour, generally brownish yellow. These characteristics, however, are due to certain impurities; in a really pure condition most oils have scarcely any characteristic taste, odour, colour, or physiological influence. In a few cases only they have special properties which appear to be inseparable characteristics, such as the purgative principle of castor oil, croton oil, and some others. At the ordinary temperatures most vegetable oils are fluid, but a few, produced especially by tropical plants, such as palm oil, cocoa butter, Chinese tallow, Ac., are solid fats. Animal fats are for the most part solid, the oils of marine animals and neat’s-foot oil being important exceptions. The various solid fats differ greatly in consistency, and the hardness of individual samples is largely affected by the nature of the food and by the health of the animal yielding them, and by some other circumstances. The relative fluidity or solidity of the various oils and fats depends on the proportions of the three principal constituents of all oils—olein, stearin, and palmitin. The fluid oils contain olein in larger proportion, that body being itself liquid at ordinary temperatures, while solid stearin and palmitin predominate in the hard fats. The viscosity of the fluid oils also ranges between wide limits. The rate of flow of an oil, which is a matter of considerable importance in several industrial applications, is estimated by comparison with the standard rate at which water of the same temperature flows through an aperture of fixed dimensions. The most viscid of the fluid oils is castor oil, which at a temperature of 15° C. is more than two hundred times thicker or more slow-flowing than water. Olive oil at the same temperature is more than twenty times thicker, and linseed and hemp oils, though among the most limpid of fixed oils, still flow about ten times as slowly as water.

Oils communicate to paper and like substances a stain which remains an irremovable translucent grease spot. They are almost entirely insoluble in water, and, excepting croton and castor oils, in cold alcohol, but in boiling alcohol they dissolve more freely, and they are perfectly soluble in ether, bisulphide of carbon, chloroform, benzol, and light petroleum spirit. In their pure condition they are neutral bodies, but, on their becoming rancid, free fatty acids are developed which give them an acid reaction. Exposed to air they absorb oxygen freely, and the class containing linoleic acid, known as drying oils, of which linseed oil is the type, thereby harden into a solid translucent semielastic caoutchouc-like body, a property of the utmost value in the arts. When they are exposed in thin layers over a great surface the absorption of oxygen proceeds with such energy that heat is evolved sufficient to produce spontaneous combustion, a circumstance frequently exemplified in the heating and igniting of heaps of oily cotton waste. The non-drying oils also on exposure to air thicken and become greasy; they acquire the peculiar disagreeable smell and acrid taste known as rancidity, owing to a kind of fermentation being set up in them through the agency of impurities, whereby the fixed fatty acids they contain are decomposed, and odorous volatile fatty acids formed by oxidation at their expense.

The specific gravity of all oils is lower than that of water, ranging from '900 in the case of cocoa butter to ∙970, the specific gravity of castor oil. Most fluid oils have a specific gravity between ’ 915 and ∙930. The specific gravity of oils varies with the temperature far more than is the case with water. It is found that for each degree Centigrade rise of temperature whale oil increases in volume 1 per 1000, rape oil 0 ’ 89, and olive oil 0’83.

When a solid fat is heated slowly till it melts and is allowed gradually to cool, it remains fluid till it falls considerably under the temperature at which it melted, and at the moment of solidification there is a sensible increase in its temperature. Butter, for example, melts at 30° to 31 0, 5 C., but does not resolidify till it falls to 19° or 20° C. These phenomena have been investigated in the case of the pure fats stearin and palmitin by Duffy (Chem. Soc. Qu. J., V. 197), who finds that these bodies undergo with great readiness three isomeric modifications, each having a distinct melting point widely apart from each other (stearin from beef giving 5Γ, 63°, and 67°), the solidifying point being slightly under the lowest of the three. The freezing point of the ordinary fluid oils ranges down to from - 27° to - 28 β C. for hemp oil, nut oil, and linseed oil, while olive oil solidifies at + 2° to 4° C. Fluid oils heated to from 280° to 300° C., and solid fats to from 300° to 325° C., undergo destructive distillation, resolving into a mixture of rich inflammable gases and a peculiarly irritating acrid vapour, acrolein.

Oils and fats are compounds of carbon, hydrogen, and oxygen, in proportions ranging, as a rule, for carbon between 76 and 80 per cent., hydrogen from 11 to 13 per cent., and oxygen from 10 to 12 per cent. Their proximate constitution was first demonstrated by Chevreul, who indeed, in the great series of classical researches embodied in his Recherches sur les corps gras iïorigine animale (1823), established the modern chemistry of oils.

The phenomena of saponification, as exemplified on a great scale in the important industry of soap-making, furnished the key for discovering the intimate constitution of oils. Oils and fats treated with alkalis, alkaline earths, and basic metallic oxides in presence of water undergo decomposition and enter new combinations. A soap is formed by the union of the alkaline body with acid constituents of the oil, known as fatty acids, ajιd the sweet body, glycerin, is liberated. The saponification of stearin with sodic hydrate, for example, may be thus represented :—

c'⅛° h } o, + 3 g≈} o = 3 ⅛⅞θ } o + c ¾"'} 0,.

Stearin. Sodic Stearate of Glycerin, hydrate, sodium (soap).

By heating oil with steam under a pressure of from 10 to 12 atmospheres, or with water superheated to about 220° C., the oil is decomposed into free fatty acid and glycerin. Thus, again taking the simple fat stearin, we have :—

⅛⅝ o 4o 3 + 3 ¾ o = 3c ½θ} 0 + Cs §'"}0 3 .

Stearin. Water. Stearic acid. Glycerin.

In the above reactions it will be observed that three molecules of water are required for the formation of free stearic acid and glycerin from one molecule of stearin, and to that extent the resulting products are heavier than the original. Beading the equation in the inverse manner we find the formation of stearin takes place by the substitution of the three acid residues of stearic acid C 18 H 35 O for the three hydroxyls H. O in the molecule of glycerin. No fat or oil is found in nature consisting of a single chemical fat such as stearin alone. All are mixtures of at least two and for the most part three or·more simple fats or glycerides of fatty acids closely allied in nature and constitution. These glycerides or combinations of glycerin and fatty acids are in their chemical relations ethers. Glycerin itself is a triatomic alcohol, and bears to the fatty acids and resulting ethers the same relation which a basic substance bears to an acid and to the salt which results from their combination. In all natural fats glycerin combines, as in stearin, by having substituted for its three replaceable hydrogen atoms three equivalents of fatty acids, whence the natural simple fats are all triacid compounds — tristearin (θi8¾5θ)3 j. O 3 , tripalmitin (θi6¾1θ)310 3j triolein (θi8¾O9 3 j. 0 3 , &c., though commonly called stearin, palmitin, and olein, &c. (For further information as to the constitution of glycerides see Glycerin, vol. x. p. 697.) The three simple fats above named form by far the largest and most important constituents of all oils and fats, the only others which bulk largely being the glycerides of linoleic acid in drying oils, and of physetoleie acid characteristic of marine oils. The number of fatty acids found combined with glycerin in oils is, however, very considerable. They constitute members of homologous series, the first or stearic series of which possess the common formula C n H 2n O s . Belonging to it are the following :— [table]

Another, the oleic series, contains two atoms less of hydrogen than the allied stearic scries, having the general formula C n H 2n . 2 O 2 . To it belong the following :— [table]

To these there remain to be added, not members of either series, two important fatty acids, —linoleic acid, C 16 H 28 O 2 , characteristic of linseed oil and other drying oils, and ricinoleic acid C 18 H 34 O 3 , the chief product of the saponification of castor oil. Among saponifiable bodies the true waxes are distinguished from other solid fats by containing no glycerin. They are principally ethers of the higher monatomic solid alcohols—cetylic and cerylic alcohol, &c. Thus spermaceti, the solid wax obtained from the head matter of the r CHOI

sperm whale, is a cetyl palmitate Certain of the vegetable waxes— e.g., Japanese wax—contain some proportion of glycerides.

Extraction of Oil.

The ordinary method for separating vegetable oils and fats from the nuts, seeds, &c., of which they form constituent parts is by pressure, with or without the assistance of heat. They are also obtained by the agency of solvents, principally by the use of bisulphide of carbon and the light petroleum spirit benzin, these being methods of production of comparatively recent introduction. Animal oils and fats are principally isolated by simple melting or “rendering” by heat. The degrees of heat and pressure necessary for separating the several fats vary very much with the fluidity of the oils themselves, the proportion in which they are present in the substances, and the nature and consistence of the associated materials. Sperm oil is indeed obtained direct in its fluid condition from the head of the sperm whale. Virgin olive oil is obtained with the gentlest pressure, and palm oil and several other vegetable fats and waxes are liberated by the agency of boiling water.

Vegetable Oil Pressing.— Pliny describes in detail the apparatus and processes used for obtaining olive oil among his Roman contemporaries, by which it appears that they derived a knowledge of the screw press from the Greeks, and applied it to the pressure of oil from pulped olives. In the East, where vegetable oil forms a most important article for food and for other personal and domestic purposes, various ingenious applications of lever presses and of combined lever and wedge presses have been in use from the earliest times. The Chinese employ the same series of operations which are followed in the most advanced oil mills of modern times, viz., bruising and reducing the seeds to meal under an edge-stone, heating the meal in an open pan, and pressing out the oil in a wedge press in which the wedges are driven home by hand hammers. The apparatus used in Europe in modern times for the extraction of oil by pressure consists of forms of the screw press, the Dutch or stamper press, and the hydraulic press. With the screw press, even of the most improved form, the amount of pressure practically obtainable is limited from the failure of its parts under the severe inelastic strain which can be put on it. It is on this account only used in the pressure of olives and of animal fats, where the power necessary is not great. The Dutch or stamper press, which has played an important part in the oil industry, was invented, as its name indicates, in Holland in the 17th century. The invention of the hydraulic press in 1795 effected the greatest revolution in the oil industry, bringing a new, easily controlled, and almost unlimited source of power into play, and on the great scale that apparatus has practically superseded all other means of pressing.

The sequence of operations in treating oil-seeds for the separation of their contained oils is ordinarily as follows :—(1) the crushing and grinding of the seed or other substance, (2) heating the oleaginous meal so prepared, and (3) expression of the oil by mechanical power.

Grinding.— As a preliminary operation oil-seeds are freed from dust, sand, and other impurities by sifting in an inclined revolving cylinder or screening machine, covered with woven wire having meshes varying according to the size and nature of the seed operated upon. In earlier times the seeds were pounded to meal by means of stamper mills. These consisted of a series of heavy wooden stampers or pestles made to rise and fall by the action of cams or wypers fixed on a revolving shaft, a pair of such stampers falling alternately with heavy force into an egg-shaped mortar about two-thirds filled with seed. As the process proceeded the material became heated, and from time to time had to be sprinkled with water. Stampers are now seen only in small old-fashioned establishments. In a modern oil-mill the screened seed is passed through crushing rollers to bruise or open the husk. The crushing rollers consist of a pair of cast-iron rollers horizontally mounted, commonly of unequal size, the larger being 4 feet in diameter, while the smaller is about 1 foot The larger roller is revolved by power and the smaller moves by simple friction against the other. Between these rollers the seed is fed by a hopper, and in passing through it is bruised and broken and so prepared for the thorough grinding it receives under the revolving edge-stones to which it next passes. These are a pair of circular stones having a diameter of about 7 feet with a thickness at their running edge of 16 inches, each weighing from 2⅜ to 3⅜ tons. They are made of very compact limestone, granite, or fine-grained sandstone, and are mounted on a vertical driving shaft, to which they are attached by a horizontal axle passing through their centre. They revolve on a bed of similar hard compact stone, and the compound rubbing and bruising effect of their rotary motion quickly reduces the bruised seeds to a fine meal. The stones are provided with sweepers, which in their revolution bring the material pressed out towards the side again into their path, and there is a separate sweeper for clearing out the finished meal from the bed of the machine by way of a slide or door provided in the side. The edge-stones revolve about twenty times per minute, and a charge of seed which is slightly moistened during the process is sufficiently ground on an average in about twenty-five minutes.

Heating.— In dealing with certain oils which are easily separated, and especially with oils used in cookery and otherwise consumed, where it is desirable to preserve the pleasant, bland, and faint fruity or nutty taste, the ground oleaginous meal is taken direct to the press and pressed for cold-drawn or virgin oil. The cake from such cold pressing, as it still retains a large proportion of oil, is subsequently broken up, reduced to meal, and heated; after which it is again subjected to pressure to obtain a further flow of oil. Ordinarily, however, the meal is artificially heated previous to any pressure, and it depends greatly on the nature of the seed and the individual manufacturer’s method of working whether the material is fully pressed at first or twice submitted to the operation. The warming of the seed meal renders the contained oil more fluid and consequently more readily separable with moderate pressure. It also enables the oil-presser to obtain a larger proportion of the contained oil, coagulates and holds back the albuminous constituents of the seeds, and similarly dries and retains mucilaginous matter. On the other hand, oil from heated meal usually is more highly coloured and harsher to the taste than cold-drawn oil; and the quality is seriously deteriorated if by chance the heat applied should exceed at most 80° C∙ The heat is applied either in open shallow iron kettles or pans heated over a direct fire or through a sand-bath; but preferably, and now generally, the meal is heated by steam circulating freely between the casings of a jacketed or double-walled pan or pans. Mechanical stirrers are kept in continuous rotation within the pan, to ensure a uniform warming effect throughout the mass. A highly-approved and convenient form of heating apparatus consists of a double steam kettle, one pan being placed above the other, each steam-jacketed and provided with mechanical stirrers. In this machine the heating action is continuous. The meal is first heated for ten or fifteen minutes in the upper pan, which is closed over with a sheet-iron cover, after which a slide in the bottom of the pan is opened and the charge is shot down into the lower pan, where it is raised to the full heat, while the upper pan is again recharged and worked up. When fully heated in the lower pan the charge is swept out at a door in the side of the pan by the action of the mechanical stirrers, and falling into a funnel is passed in measured quantities direct into bags, and without delay prepared for and placed in the press. The kettles are of a capacity sufficient to heat seed for charging three single presses at each operation. A form of heating kettle is also now in use in which the object is effected by direct injection of steam into the mass, whereby the meal is not only heated but a beneficial amount of moisture is distributed throughout the material. In mills of the most recent construction such steam kettles are used in connexion with an improved form of crushing rollers,—employment of edge-mills being dispensed with. These rollers consist of a series of four or five chilled iron or steel cylinders mounted in vertical order like the bowls of a calender. The seeds pass in succession between the first and second rollers in the series, then between the second and third, and so on till they are delivered by the lowest, sufficiently bruised, crushed, and ground.

Pressing.— With the least possible delay the meal is transferred from the heating kettles, so that the oil may be pressed out while the material still retains its heat. Measured quantities, say 10 to 12 lb of meal, are filled into woollen bags of strong, thick texture, sufficiently open and porous to allow...